2.0 Summary of previous research on the Great Plains and Cedar Hills aquifers in the studey area

Studies focusing on the stratigraphy and depositional environments
of the Lower Cretaceous Series have been conducted along the
outcrop and in the subsurface of Kansas, Nebraska, and Colorado
since the late 1800's. Only the most recent of these investigations
will be mentioned here in this short review. Excellent summaries of
the nomenclatural history of the Lower Cretaceous in Kansas are
given by Latta (1946), Merriam (1963), and Franks (1966).

Since the Dakota Group was defined by Meek and Hayden (1861) the
name has been applied somewhat indiscriminantly to various
sedimentary sequences composed of Lower Cretaceous sandstones
and shales that may not all be time equivalent. Most of these early
works have largely dealt with rocks exposed at the surface. The
Cheyenne and Kiowa have been recognized as formations since they
were defined by Cragin in 1889 and 1894, respectively (Latta, 1946).
The Dakota has been officially recognized as a formation by the
Kansas Geological Survey since 1942 when the term was restricted to
cover the sequence between the base of the Graneros Shale and the
top of the Kiowa Formation. Plummer and Romary (1942) subdivided
the Dakota Formation into two members, the Janssen Clay member
above and the Terra Cotta Clay member below, based on field work
along the outcrop in central Kansas.

With the discovery of oil and the need to dispose of the produced
brines, interest shifted to the subsurface. A reconnaissance
investigation by Frye and Brazil (1943) was conducted to determine
the hydrogeology of the unconsolidated and bedrock aquifers in Ellis
and Russell counties. They mapped what they believed to be the
easternmost extent of the Cheyenne Sandstone in western Russell
County. Later, Swineford and Williams (1945) conducted a geologic
and hydrologic investigation of the Lower Cretaceous in the same
area at the request of the State Board of Health. Concern was
expressed that insufficient geologic criteria were being used to
approve the shallow disposal of oil-field brines in the Cheyenne
Sandstone. Swineford and Williams examined drill cuttings and the
light and dark fractions of insoluble residues in an attempt to
differentiate the Cheyenne Sandstone from adjacent formations.
They found that differences in grain size and roundness of the
sandstone grains and the composition of the insoluble residue suite
could be used to locate formational boundaries. Moreover, they
revised the earlier work of Frye and Brazil by concluding that the
Cheyenne Sandstone extends farther east of western Russell County
than the earlier investigation had indicated.

Much later, Merriam (1957) renamed the Dakota Formation the
Omadi in an attempt to correlate the Kansas Lower Cretaceous rock
section with the same section as defined farther north in Nebraska.
He defined three members which are, from oldest to youngest, the
Cruise, the Huntsman, and the Gurley. In this scheme, the Dakota was
elevated to group status including, from oldest to youngest, the
Cheyenne, the Kiowa, and the Omadi Formations. These formational
names were applied to the subsurface by Merriam (1957; 1963) and
used to show the stratigraphic relationships between the various
Mesozoic-age rock units in the subsurface of western Kansas. This
terminology has not been adopted by the Kansas Geological Survey.

Our present understanding of the stratigraphy and depositional
environments in the Lower Cretaceous comes from fieldwork that
was done along the central and southern Kansas outcrop in the
decades of the 1960's, and 1970's. Franks (1966) mapped the Dakota
and Kiowa Formations along the outcrop. Field mapping shows the
contact between the Dakota and Kiowa Formations to be locally
disconformable. In some cases, sharp contacts abruptly separate clay
rocks of the Dakota Formation from sandstones and clay rocks in the
Kiowa and, in other instances, basal sandstones in the Dakota rest on
interbedded sandstones and clay rocks in the Kiowa. Franks has
interpreted the upper portion of the Kiowa to be part of a regressive
sequence of interbedded shales and sandstones. The upward increase
in grain size and increase in kaolinite in the Kiowa marks the onset of
regressive conditions. At the base of the Dakota, the occurrence of
conglomeratic sandstones and red-mottled clay rocks signifies the
development of alluvial depositional systems. Previously, Twenhofel
(1920, 1924) and Tester (1931) had concluded that the two
formations were laterally and vertically conformable and
gradational. This conclusion was based on the occurrence of Kiowa
sandstones containing leaf fossils similar to those found in the Dakota
strata. The concept of the intertonguing of the two units led to the
development of a large scale deltaic model of sedimentation
involving the two formations.

Using the results of Scott (1970), Franks (1975, 1980) proposed a
transgressive-regressive model of sedimentation to account for the
distribution of lithofacies seen in the Cheyenne, Kiowa, and Dakota
sequence of formations in Kansas. The Cheyenne Sandstone and
Kiowa Formation record a northeastward transgression of the sea
across Kansas. Sedimentation was governed by the type and rate of
sediment supplied to the interior seaway and the rate of subsidence
during early Kiowa time. These rocks were deposited in and near the
eastern margins of the Early Cretaceous epicontinental sea that
spread over the southern Western Interior.

Franks' (1975, 1980) interpretation of the geologic framework shows
a progressive overlapping of older Lower Cretaceous stratigraphic
units on the Cretaceous-Permian boundary from southwest to
northeast across the Kansas outcrop. Accordingly, the Cheyenne
Sandstone is present only in southern Kansas and does not extend in
the subsurface very far northward of the outcrop. This contradicts
the earlier work of Frye and Brazil (1943) and Swineford and
Williams (1945) in the Ellis-Russell County area and Merriam (1957).
These authors show that the Cheyenne Sandstone is present in much
of the subsurface of western Kansas. However, Franks (1975, 1979
and 1980) believes that the lower part of the Kiowa Formation, the
so-called "Longford member," resembles and has been confused with
the Cheyenne Sandstone. These rock units are not time equivalent
even though they are similar in aspect (Scott, 1970). Farther north
the Kiowa Formation rests directly on the Cretaceous-Permian
boundary. Near the Nebraska-Kansas border, in the north central
part of the state, the Kiowa is not present and the Dakota rests
directly on the Lower Permian.

Farther up the stratigraphic column in the upper part of the Dakota
Formation, clay rocks of the Janssen member contain lenticular
channel sandstones of the Rocktown Channel sandstone in
north-central Kansas (Rubey and Bass, 1925). These rocks are
interpreted as having been deposited in a deltaic complex under the
influence of a transgressing Graneros sea (Hattin, 1965; Franks, 1965,
1975; Siemers, 1971, 1976). The Rocktown Channel sandstone body
has been divided by Siemers into two major subfacies, grading
within approximately 30 miles from a highly sinuous distributary
channel sandstone into estuarine and delta-front sandstones. A
lower cross-bedded sandstone subfacies is 54 to 66 feet thick and
900 to 1800 feet wide and is more less confined to a sinuous,
elongated v-shaped trough whereas the upper flat-bedded subfacies
forms a tabular-shaped unit. These rocks reflect an increasingly
marine character in the style of sedimentation in the upper part of
the Dakota westward of the outcrop as the Dakota intertongues with
the Graneros Shale in northwest Kansas (Hattin and Siemers, 1987).

2.2 Review of Previous Hydrogeologic Investigations of the Great Plains and Cedar Hills Aquifers in the Study Area

Hydrogeologic investigations of the Great Plains aquifer have
concentrated on locating potential surface and ground-water supplies
for municipal, domestic and stock uses and on the effects of
oil-industry activities on water quality. These include Frye and
Brazil (1943), Swineford and Williams (1945), Leonard and Berry
(1961), and county reports by Latta (1950), Hodson (1965), and
McNellis (1973).

One of the earliest of these studies was conducted by Frye and Brazil
(1943) in the oil-field areas of Ellis and Russell Counties. Their
objective was to evaluate the resource potential of surface and
ground waters and to assess the effects of oil-field activities on water
quality. During the course of the study, several test holes were
drilled into the Dakota and Kiowa Formations and the Cheyenne
Sandstone in Russell and Ellis County. As the drilling of each test
hole progressed, water samples and hydraulic head measurements
were taken. The unpublished data from the test-hole drilling
program shows that the vertical direction of ground-water flow is
upward from the deeper aquifers in the Lower Cretaceous to shallow
aquifers. The authors note that the chemical quality of ground
waters in the Dakota is highly variable laterally and vertically.

A water sample collected from a deeper zone in the Lower
Cretaceous in one of the test holes was analyzed and contained
22,900 mg/l chloride. However, a water sample taken from a nearby
well in the Dakota contained a chloride concentration of only 23
mg/l. Additionally, water level measurements were taken in wells
obtaining water from the Dakota and a map was prepared to show
the potentiometric surface of the upper part of the Dakota in T14-
15S between R12-15W. The map clearly shows that ground waters
from the upper part of the Dakota discharge to the Smoky Hill River
in Russell County. As a part of their investigation, Frye and Brazil
looked into the brine-disposal practices of the oil industry in Russell
and Ellis Counties. They found that oil-field brines were being
disposed in evaporation pits, shallow disposal wells drilled into the
sandstones of Lower Cretaceous age (including the lower part of the
Dakota Formation), and deep disposal wells drilled into porous rocks
of Pennsylvanian age or older. They concluded that disposal of
oil-field brines could be done safely in the Cheyenne Sandstone since
sandstones in the Cheyenne constituted a stratigraphic trap. In
contrast, sandstones in the Dakota were found not to be suitable
because brines injected into the lower portions of the Dakota
Formation would migrate upwards into the fresh water zones
through interconnecting sandstone lenses.

Somewhat later, Swineford and Williams (1945) investigated the
vertical variation of water quality in the Lower Cretaceous and
Lower Permian in western Russell County, where Frye and Brazil
(1943) had conducted their earlier investigation. During the drilling
of test holes through the Lower Cretaceous and Lower Permian rocks,
they collected water samples in order to determine the vertical
variation of water quality in each borehole. They found a
progressive increase in total dissolved solids (TDS) and chloride
content with depth. Ground waters in the upper part of the Dakota
were found to contain less than 9000 ppm TDS. In contrast, the TDS
content of ground waters from the lower part of the Dakota
Formation ranged from 9,000 to 61,000 mg/l. In the lower units the
concentration of TDS in ground waters from the Kiowa and Cheyenne
ranged from 33,000 to 62,000 mg/l and from 31,000 to 71,000 mg/l
in the Permian red beds (Cedar Hills Sandstone). Figure 2 shows schematically the variation of TDS with depth considering all of the samples collected.

Figure 2. Composite water quality profile of the
Lower Cretaceous and Permian bedrock aquifers in Russell County,
generalized from test drilling. Adapted from Swineford and
Williams (1945).

Most importantly, the authors were able to delineate hydrochemical
zones in these aquifer units based on an interpretation of the water
chemistry. They found that all of the water samples from the upper
half and some from the lower half of the Dakota were distinctly
different in constituent composition from those samples that came
from the Kiowa, Cheyenne, and Cedar Hills. The authors interpret this
variation in water chemistry to be the result of the mixing of sodium
chloride brines from the lower units with formation waters in the
lower half of the Dakota Formation. This is discussed further in
section 6.2.3.

Noting the differences in water chemistry of the Lower Dakota
between the oil-field areas and those areas on the edge of or away
from the oil fields, Swineford and Williams also concluded that
ground waters from the lower part of the Dakota have been affected
by disposal of oil-field brines. They speculated that this could have
occurred as a direct result of induced migration upwards from lower
injection horizons in the Cheyenne and Cedar Hills Sandstones caused
by the "pressuring up" of injection horizons or through unplugged or
improperly plugged boreholes in the oil-field areas.

Later work by Latta (1948), Jordan et al. (1964), and Hargadine et al.
(1979) found that mineralized waters from the Dakota Formation
eventually discharge into the alluvial aquifers and surface waters of
the Smoky Hill, Saline, and Solomon Rivers in north-central Kansas.

Concern for the protection of ground waters in the Dakota from
contamination by oil-related activities and the lack of information
about water quality in the Lower Cretaceous rocks (Great Plains
aquifer system) has prompted several reconnaissance-level surveys
using log analysis techniques to derive the salinity of ground water
in northwest Kansas. Early work by the Kansas Geological Survey
produced relative salinity maps of the upper and lower sandstones in
the Cheyenne Sandstone (Anonymous, 1960). Relative salinity of the
waters was calculated from a combination of SP, and long and short
normal resistivity logs. The relative salinity was calculated because
water samples from the Cheyenne Sandstone were not available to
calibrate the calculated results. The results of this
reconnaissance-level effort show considerable variability in
ground-water salinity laterally and vertically in northwest Kansas.
However, the author could not attribute the cause of this variation.
Much later, Malone (circa 1984-85) produced a similar map showing
the salinity of ground waters in the Dakota of northwest Kansas for
Murfin Drilling Company, Wichita, Kansas. The map shows relatively
fresh waters in the Dakota for Logan, Graham, Sheridan and parts of
Decatur, Trego, and Norton counties. Waters in the Dakota are
relatively saline in Cheyenne, Rawlins, Sherman, Thomas, and
Wallace counties.

Several county and subregional reconnaissance studies that have
dealt in part with the Great Plains aquifer system in the study area
have also been completed. These include reports on southeastern
Trego, southern Ellis, and northern Rush counties in the Smoky Hill
valley (Leonard and Berry, 1961), Rush County (McNellis, 1973), and
Trego County (Hodson, 1965). Within these areas the authors report
widespread use of aquifers in the Dakota Formation for domestic and
stock uses where other supplies are not available.

Perhaps the most important regional study of the Great Plains
aquifer system has been conducted recently under the U.S. Geological
Survey's CM RASA Program (Helgeson et al. in review). Using
existing data, their study puts the hydrogeology and water quality of
the Great Plains aquifer system in a regional perspective that covers
parts of Kansas, Nebraska, Wyoming, Colorado, New Mexico, and
Oklahoma. In an evaluation of the Lower Cretaceous bedrock aquifer
system, they note that variation in the water chemistry of the Great
Plains aquifer appears to be related largely to the degree that the
aquifer has been flushed of formation waters. Calcium-bicarbonate
and mixed cation-mixed anion type waters are found near the
outcrop areas. Sodium-chloride type waters are contained in the
aquifer either where these rocks are more deeply buried and have
not been flushed by recharge waters, or where they are being
recharged by saline waters from the underlying Permian.